Inkjet recording apparatus are capable of printing high-quality letters at a high speed and at a low cost. The apparatus are employed in copy machines, facsimile machines, printers and word-processors, and are thus widely used as information recording apparatus in offices as well as for personal use. Various techniques have been proposed to improve the inkjet recording apparatus, and nowadays these techniques still focus on higher speed recording, higher resolution, and full-color printing.
The following recording methods are available in inkjet recording: a method employing an electro-thermal transducing element, such as a heating resistor, as an energy-generating-means for expelling color material; a method employing an electromechanical transducing element such as a piezo element; an electrostatic method employing electrical energy as it is; and the like. Regarding a recording head employed in the apparatus, a serial scanning head has been commercialized. This head is mounted to a carriage and movable in a direction (main scanning direction) orthogonal to a transfer direction of recording paper (sub-scanning direction).
As a means for high speed recording, a dc motor is employed in the apparatus as a transfer motor for driving a transfer roller which transfers recording paper. This dc motor provides for easy speed adjustment and relatively large torque. A rotary detector is provided for detecting a rotating angle of the transfer motor in order to correctly control the feeding amount of the recording paper. On the rotary detector, radially extending slits are formed along the entire rim of the rotary detector at equal intervals.
The rotary detector is disposed coaxially with any gear (rotating member) of a transmission gear-train (driving force transmitting machine) which is placed between the transfer motor and the transfer roller. The transmission gear-train transmits the driving force of the transfer motor to the transfer roller.
The structure of the conventional inkjet recording apparatus discussed above has the following problems. The first problem relates to damage of the rotary detector. The rotary detector is often formed of thin plastic members. Such a delicate detector is vulnerable to being damaged due to a careless mistake by an operator at an apparatus assembly line, e.g., a shock by collision with another member. If the rotary detector is damaged, it is impossible to detect a rotating angle with a detecting sensor, or if the rotary detector is deformed, the rotating face of the detector shakes, and it is impossible to detect a correct rotating angle.
The second problem refers to stains on the rotary detector. The rotary detector can detect a rotating angle at greater accuracy with a larger diameter. However, the larger diameter for the greater accuracy prevents the apparatus from being downsized. When a high-quality letter is printed at a high speed, the recording head moves rapidly and the nozzle ejects smaller amount of ink, and thus the ink tends to scatter. Then the scattered ink attaches to the rotary detector, thereby producing an error in detecting a rotating angle. A smaller diameter of the rotary detector would avoid this problem; however, the smaller detector would produce another problem, i.e., lowering the accuracy of detecting a rotating angle.
The third problem relates to eccentricity of the rotary detector. The rotary detector must be mounted to a rotating shaft without eccentricity both in radial and thrust directions. Therefore, it is preferable to be to able check the eccentricity of the detector with ease.
The fourth problem relates to the size of the rotary detector. For detecting a rotating angle of the transfer motor, it is required to optically detect, with a detecting sensor, a number of slits of the rotary detector as the rotary detector is rotated by the transfer motor. When the detecting sensor is a transmission type sensor, its light-emitting-section and light-receiving-section are placed at both sides of the rotary detector, and the detecting sensor is mounted to the rim of the rotary detector. As a result, the detecting sensor protrudes largely in the radial direction compared with the gear disposed coaxially with the rotary detector. This structure is not preferable because it goes against the goal of downsizing the apparatus.